Process of coating aluminum



Patented July 26 y 932 HELMER BENGS'I'ON, OF INDIANAPOLIS, INDIANA,ASSIGNOR, BY MFSNE ASSIGNMENTS,

TO ALUMINUM COLORS INCORPORATED, OE INDIANAPOLIS,

TION 10F INDIANA INDIANA, A CORPORA- PROCESS COATINQ'ALUHINUM NoDrawlng.Application filed June 23, 1930, Serial No. 463,851. Renewed May 10,1932.

This invention relates toprocesses of coating aluminum; and 1t comprisesa method of providing aluminum or aluminum alloys with a. decorative,hard, corrosion-resisting coating layer containing A1 S0 and H 0, saidcoating being adapted to be dyed, lacquered and otherwise treated,wherein an article of aluminum or aluminum alloy is temporarily made'ananode in rather strong tween 1.5 to 1.7 and the acid advantageouslycontaining a considerable proportion of glycerin or another inhibitor,all as more fully hereinafter set forth and as claimed.

As is well known, ordinary aluminum articles are covered with a hardcoherent coating of aluminum oxid formed by the action of air. Thiscoating is self-renewing in the sense that Where it is removed, cracked01- broken it is, practically at once, renewed by the action of the air.While this coati'n is excellent protection against corrosion an is notunsightly, its existence precludes the ordinary decorative treatmentswhich can be :1 applied to other metals. It is, for example, dilficultto make different finishes by different ways of buffing or polishing;and the coating does not give good anchorage to lacquers and varnishes.For this reason, aluminum articles are seldom decorated in any way.Plating is difficult. v

It is an object of this invention to obviate difiiculties in decoratingarticles made of aluminum and aluminum alloys by substituting for theordinary oxid film a special coating produced by anodic treatment of thealuminum in a special way. It has been found that if an articleofaluminum, or of aluminum alloy, is thoroughly cleaned and buffed andis then made an anode in strong sulfuric acid of 1.5 to 1.7 specificgravity there develops a hard coating of substantial thickness. Thecoating is insoluble in water, contains sulfuric" acid in combinationwith the alumina in an insoluble form and has an empirical compositioncorresponding to that of a highly basic hydrated sulfate of alumina. Itis minutely pervious but the porosity is sub-microscopic in characterand the pores are readily sealed to give a completely prosulfuric acid,the specific gravity being be-.

tective weather-proof layer. This coating is colorless but, as I havefound, thecontained material'is in such a reactive form as to take upand readily hold those dyes which are customarily used with mordants. Inother words, the alumina compound can be converted into a lake of anydesired color or hue.

Decorative on the coating produced as described'may be achieved in anumber of other ways. Its thickness is sufficient to permit bulfing andmechanical finishing. And it is found that in spite of the minutecharacter of the pores, it will readily take up oils'and oily materials.This readypermeability by oils is possibly connected with the adsorbentnature of=thematerial of the coating; a view which is supported by thefact that oils once taken up are removed with difliculty by gaso--- lineand other oil solvents. Colored oils give various decorative efl'ects.

The strength of the sulfuric acid used in the electrolytic treatment isof great importance. The best results are given by a sulfuric acid of aspecific gravity of 1.5 to 1.7; that is an acid containing from 60 to 77per cent of H SO A 64-65 per centacid is generally useful. Solutionssomewhat stronger and weaker than those in this range will operate formy purposes but are open to objection. With higher specific gravities,the conductivity is lessened to an extent making it difiicult to secureefficient current densities while certain lower specific gravities,those between 35 and 60 per cent acid, give too much conductivity andalso cause excessive etching and attack on the aluminum. No etching orattack on the aluminum surface is here desired other than that necessaryin forming the coating. It has been found highly advantageous, even inusing the optimum concentration of sulfuric acid, to add some glycerinto the solution. This decreases etching and attack still further. I useordinarily about 1 part by volume of, glycerin-to 15 parts by volume ofthe 1.5-1.7 acid. In the presence of the glycerin the specific gravitycan be lowered a little below the range mentioned without too muchattack and with the possibility of using higher they cause surfaceroughening and attack without production of the type of coating heredesired. The sulfuric acid should be free of HCl or chlorids, as thesecause too much solution loss.

In operation, the apparatus in which the anodic coating is effected ismost conveniently made of stoneware or other ceramic or vitreousmaterial. However, containers of wood or metal, lined with materialsresistably at 25 ant to sulfuric acid, such as pitch, rubber or sheetlead, may be used. The cathodes may be of any convenient material, but Ifind cathodes of ordinary chemical lead best adapted. The size and shapeof the oathodes, of course, vary according to the work to be done; butfor many purposes they may be located on the side walls of the bath andcover about per cent of the side wall area.

With lead lined containers the lead lining may very conveniently serveas a. cathode. The articles to be treated may be hung 1n the bath uponthe usual conducting rods. Pointed clamps may be used with sufficientpassage of current so that the uncoated area in the finished article ismerely an imperceptible spot. The direct current voltage used isordinarily about 12 volts between terminals. With a voltage of thisorder no rheostats or regulating devices are necessary. The bath iscooled in any of the ordinary ways. With large baths, cooling coils orfcooling jackets are generally desirable. Circulation of theelectrolyte, with agitation thereof, in well known ways, is alsodesirable. Heat is evolved in the operations. It is best to keep thetemperature between 20 to 30, prefer- C. The anode current density,

- with the usual spacing between anodes and cathodes, is ordinarilybetween 6 and 10 amperes per square foot of surface of the article beingtreated.

The aluminum articles to be coated should be subjected to the usualpreliminary cleaning treatments. Where a lustrous coating is wanted thealuminum should be'butfed before dipping. In any event, grease and otherforeign matter should be removed.

For most purposes the time required for the anodic treatment to producethe desired coatingis 10 to 30 minutes. However, a useful layer may beformed in 5 minutes or less and of course an extended time of an hour ormore does no harm. Flow of current decreases with the increase inthickness of the coating and after a' normal coating has been formed,prolongation of treatment efiects no material change. Coating isuniformat all of dissolved sulfate of alumina and this can be furtherreduced by the use of inhibitors. For the most part, the aluminumoxidized in the anodic treatment reappears in the coating. Such aluminumas does dissolve forms aluminum sulfate which after a time crystallizesout.

If the work is not completely clean there is apt to be uneven coloringdueto varying thicknesses of coating. In such an event, it

is best to remove the coating and reprocess the article. For removingthe coating a 5 to 10 per cent hydrofluoric acid dip is mostsatisfactory. On immersion the coating rapidly dissolves, solutiontaking perhaps 15 seconds. There is then a vigorous evolution of gas asthe acid reaches the metal. When this occurs, the metal is removed,washed and is ready for processing.

After treatment the article isremoved, rinsed and dried. The coating isinsoluble in Water at the ordinary temperature and is not injured byrinsing. Thorough rinsing to remove contained electrolyte is desirable.The wet coating after rinsing may be dyed by treatment with a solutionof a dye; the dye being taken up to form what may be termed a lake. Thedye is found to penetratethe coating and give uniformity of color at allpoints. Exhaustion of the dye bath is usually good- The coating may bebuffed and mechanically finished, before or after the application ofdye. Alternatively, in lieu of dyeing the coating it may be oiled With asuitable oil such as lanolin or lard oil. Neutral petroleum oils andwaxes may also be used. Apparently the oil is taken up in the body ofthe coating and not merely supen ficially. In bufiing, the coatingusually takes up oily matter from the bufiing compound. The dyed coatingis usually dried and buffed or otherwise finished. It may be oiled.

The anodic treatment described usually gives a coating of about 0.0005inch thick. With a somewhat greater time of anodic treatment thethickness of coating may be 0.0015 inch or more; These thicknesses aresuflicient for all ordinary purposes, and while somewhat thickercoatings may be secured the result is more attack on the underlyingaluminum The coating thickness varies directly with the potential of thebath and, for a time, with sulfuric acid diluted to 1.5 to 1.7 specificgravity.

5. In electro-coating articles of aluminum and aluminum alloys, theprocess which comprises making such an article an anode in sulfuric aciddiluted to 1.5 to 1.7 specific gravity, such acid containing a littleglycerin.

6. In electro-coating' articles of aluminum and aluminum alloys, theprocess which comprises passing current between such an article andanother electrode in an electrolytic bath of sulfuric acid dilutedto 1.5to 1.7 specific gravity.

7. In electro-coatingarticlesof aluminum andaluminum alloys,.the processwhich comprises anodically coating the said articles in a bath ofsulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing, anddyeing the said coating.

8. In electro-coating articles of aluminum and aluminum alloys, the.process which comprises anodically coating the said articles in a bathof sulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing thecoating and plugging the pores thereof, whereby the article becomesresistant to corrosion.

9. In electro-coating articles of aluminum and aluminum alloys, theprocess which comprises anodically coating the said articles in a bathof sulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing thecoating, plugging the pores thereof and finally bufling the saidcoating. 1

10. In electro-coating articles of aluminum and aluminum alloys, theprocess which com prises anodically coating the said articles in a bathof sulfuric acid diluted to 1.5 to 1.7 specific gravity and thereaftermechanically finishing the article.

In testimony whereof, I have hereunto affixed my signature. 1 HELMERBENGSTON.

the duration of treatment. Thickness varies somewhat with the strengthof acid. In the solution in the acid asaluminum sulfate and after/a timethe acid becomes saturated and deposits crystals. To dispose of thiscrystallized sulfate and keep down the temperature of the bath, it isusually expedient to keep a portion of the bath in cyclic circulation toand through a filter and cooling means and back to the bath. A leadlinedpump may be used.

While I have described making the aluminum an anode in a sulfuric acidbath with employment of direct current of the usual type used inplating, an alternating current may be used in lieu of direct current.The rectifier action of aluminum makes this feasible. I have securedgood coatings with low voltage alternating current. Even 8 volts havesufiiced. In some instances this applicability of alternating current isadvantageous since a small step-down transformer can be used in lieu ofthe more complex equipment necessary in securing low voltage directcurrent from ordinary'60 cycle 110 volt alternating current.

Some properties of a coating produced by the anodic treatment are asfollows: it is a non-conductor of electricity when dry and particularlywhen oiled, evincing a good resistance against spark puncture; it isreadily reactive with, adsorbs and permanently holds organic dyes; it isresistant to water and wear; it stands bufing and takes a high polish;it withstands heating without lifting or breaking away'from the metal;it withstands punching, stamping and the like to a reasonable degree; itbinds paints, oils, enamels, lacquers and the like; it shows no definitecrystalline structure under X-ray examination; and it is a coating ofreasonable thickness rather than a film. While the coating is, asstated, amorphous or non-crystalline under the X-ray, it has a laminarstructure. Next to the metal is an extremely thin, extremely hard,glaze-like layer while there is a much thicker outer layer ofconsiderably less hardness integrally attached thereto. The hardness ofthis outer layer however is sufficient to permit bufling and polishingetc., as stated. The sub-microscopic porosity extends through bothlayers and is of a type like that with silica gels, the amount ofinternal surface being very great. This, together with the transparentnature of the coating, is an explanation of the bright and true colorswhich can be obtained in dyeing. In one particular coating about 0.00066inch thick, the porosity represented about 40 per cent of the totalfilm. The coating is resistant to heat, withstanding temperatures up to,say, 300 C.,

without shelling or cracking and without much lossof water. With wellrinsed material, the amount of hygroscopic moisture given up at 100 C.,is very little. The restof the H 0 is more permanently held. The coatingis'tolerably resistant to acids but loses sulfuric acid in alkalineliquids. It is insoluble in cold water but gives up sulfate to boilingwater. Two hours boiling with distilled water are however required forcompletion of the action. .Rinsed, dry material does not corrode in theair and with the pores plugged or filled, corrosion by sea water doesnot occur. As illustrating the adsorptive power of the coating material,it may be mentioned that on application of lacquers c0ntaining dyes,some of the dye sometimes migrates into the underlying coating.

. Aside from its advantageous mechanical properties, the present coatingowes most of its value to its highly developed adsorptive power; a powerwhich enables it .to take up i and hold oils, coloring matters, etc.against withdrawal. I attribute this high adsorptive power in part tothe large amount of fixedly contained SO or basic aluminum sulfate asthe case may be. This basic sulfate is in a form resisting action bycold water and it is extremely active as an adsorbent. In the article ofthe present invention, the fixedly held S0 may range between, say, 16percent and 23-24; per cent of the coating. Using weaker sulfuric acidthan 1.5-1.7 specific gravity, it is not possible to fix as much S0 andthe adsorptive properties are not as good.

The present application contains certain matter in common with prior andcopending application 368,742 filed J une 5, 1929, and is filed as asubstitute for and continuationin part'of said application. Aluminumcoated with the described coating produced by the present process is notspecifically claimed herein, it forming the subject matter ofanother'and copending a lication, Serial No. 460,543, filed June 11,1930.

What I claim is;

1. In electro-coating articles of aluminu and aluminum alloys, theprocess which comprises making such an article an anode in sulfuric aciddiluted to about 64-65 per cent 2. In electro-coating articles ofaluminum and aluminum alloys, the process which comprises making such anarticle an anode in sulfuric acid diluted to about 64-65 per cent H 80and containing a little glycerin.

3. In electro-coating articles of aluminumand aluminum alloys, theprocess which com-- prises making such an article an anode in sulfuricacid diluted to about 64-65 per cent H 80, and containing an inhibitor.

4. In electro-coating articles of aluminum and aluminum alloys, theprocess which comprises making such an article an anode in'

